Metal sheet having a deterministic surface structure and method for producing a formed and coated sheet-metal component

ABSTRACT

The invention relates to a sheet metal having a deterministic surface structure, the surface structure being impressed into the sheet metal, the surface structure having at least one peak region and at least one valley region, the peak region and the valley region being joined by a flank region. The invention further relates to a method for producing a formed and coated sheet-metal component.

The invention relates to a sheet metal having a deterministic surface structure, the surface structure being impressed into the sheet metal, the surface structure having at least one peak region and at least one valley region, the peak region and the valley region being joined by a flank region. The invention further relates to a method for producing a formed and coated sheet-metal component.

Zinc phosphate layers are used for the surface finishing of coated (galvanized, hot-dip aluminized) and uncoated sheet metals in order to achieve significant improvements in surface-relevant properties. This includes in particular increasing the corrosion resistance and also improving the formability and coating-material adhesion. Zinc phosphate coats are inorganic crystalline metal phosphate layers which are deposited from an aqueous phase. Rather than being continuous layers, they constitute an accumulation of individual zinc phosphate crystals, with a whole host of production factors determining the position, size, distribution, composition, and chemical and mechanical properties of these crystals. These factors include in particular the composition of the phosphating solution, the preparation of the substrate, and the process parameters during phosphating. The phosphating process is a multistage process which as well as multistage rinsing steps is made up in particular of a pretreatment step, an activation step, and at least one phosphating step.

As a result of the applied zinc phosphate crystals there is a significant increase in surface area, resulting in improved forming properties (enhanced oil retention capacity and more uniform distribution of oil). The crystals serve, furthermore, as an ideal adhesion base for coating materials.

The zinc phosphating of metal surfaces entails a high expenditure on equipment (multistage process=operations including cleaning, activating, phosphating, and rinsing; monitoring) and on energy (the individual process baths having a size of some to many cubic meters and require continual agitation and in some cases heating at up to 60° C.). Further factors of a standard zinc phosphating procedure include a very high level of chemical consumption (including disposal costs and maintenance) and also, among others, the use of the heavy metals manganese and in particular nickel (trication phosphating for increasing the temperature resistance and alkali resistance and also for grain refinement and for adjusting the hue). In the automobile segment, accordingly, there is great interest in replacing the zinc phosphating process by a more eco-friendly and operationally reliable alternative. Examples of such alternatives are nickel-free phosphating processes or silane-based systems.

In the generation of zinc phosphate coats it is necessary to ensure a sensitive process regime during activation and phosphating, in order, for example, to prevent entrainment or ageing process baths, which may have adverse consequences for the phosphating process and may lead to disrupted phosphating, more particularly to noncoherent, surface-covering zinc phosphate coats and/or to impaired coating-material adhesion. The activation may additionally react sensitively to the surfaces which have not undergone optimal cleaning, with the consequence that phosphating spots with large and small crystals lead to visual differences in the phosphating pattern (dark/light) and unphosphated areas develop. Dehydration at high temperatures may also have disadvantageous consequences, in the baking of a coating material, for example, and may thus lead to a deterioration in the coating-material adhesion.

It is the object of the invention, therefore, to specify a sheet metal and also a method for producing a formed and coated sheet-metal component, with which the expenditure involved in zinc phosphating can be reduced or removed in comparison to the prior art, with the surface having properties substantially comparable with those of a conventionally zinc-phosphated surface.

The provision of a defined surface structure on a skin-pass-roll sheet metal is essential for further processes particularly in the further-processing industry for the manufacture of sheet-metal components in the automobile segment. In the course of component manufacture, particularly in forming processes, it is advantageous for process media used, such as oil and/or lubricants, for example, to be present uniformly and in the required surface weight at areas relevant to the forming process. In order to be able to establish an extremely advantageous surface roughness on sheet metals for subsequent processing, the sheet metal is subjected to a rolling procedure (skin-pass rolling) in which, among other things, a roughness is established on the sheet metal using textured skin-pass rolls. Via the skin-pass rolling it is for example possible to eliminate and/or compensate strip corrugation, among other adverse factors, if the sheet metal has been subjected in particular beforehand to a thermal treatment (annealing, etc.). Another effect of the skin-pass rolling is to reduce the thickness and/or extent of the length between incoming and outgoing sheet/strip (degree of skin-pass rolling), thereby making it possible to tailor qualities including the mechanical properties of the sheet metal.

The inventors have determined that sheet metals having a deterministic surface structure can be produced which not only combine the aforesaid advantages but are also able to replace entirely or at least partly a conventional zinc phosphating, by creating an artificial increase in the surface area, such that the peak region and/or the valley region has a substructure which is configured in such a way that the substructure has a surface greater by at least 3% in comparison to a flat projection face of the peak region and/or of the valley region, or has an Sdr of at least 3%. In accordance with the invention the increase in surface area is generated no longer by a zinc phosphating or by zinc phosphate crystals, but rather by a larger surface area, which can be custom-tailored. The custom-tailored surface area enlargement not only serves as an optimal adhesion base for a coating of material, but may also consequently promote the suitability for adhesion, by means of a larger interface provided, with the adhesive able to be offered a corresponding reaction area.

The substructure has more particularly a surface area which is greater by at least 7%, preferably by at least 10%, preferably by at least 15%, more preferably by at least 20%, in comparison to the flat projection face of the peak region and/or of the valley region, more particularly as determined by atomic force microscopy (AFM), which enables, for example, a resolution with an area of up to 90×90 μm². Depending on the surface structure to be measured, it is possible to select a resolution in the order of magnitude, for example, of a valley region or of a part of a valley region or of a peak region or of a part of a peak region, which may also, for example, have an area of less than 90×90 μm².

A flat projection face of the peak region or valley region, respectively, refers to a planar face which can be viewed and/or determined in plan view, parallel to the sheet metal plane. The greater surface area generated by the substructure in the peak region or valley region corresponds to the actual, determinable three-dimensional surface/face.

The Sdr relates to a developed boundary value ratio and is also a measure of the surface enlargement, indicating the percentage of the additional area of a definition region that is attributable to a texture (substructure), in comparison to the absolutely planar definition region, with the definition region (resolution) being able to be directed at a part of the valley region or at a valley region and/or at a part of the peak region or at a peak region. The substructure more particularly has an Sdr of at least 7%, preferably of at least 10%, preferably of at least 15%, more preferably of at least 20%. A flat surface has an Sdr of zero. The Sdr for example is also determinable by or by means of atomic force microscopy (AFM).

A deterministic surface structure is understood to refer to recurring structures (at least one valley region or valley regions and at least one peak region) which have a defined shape and/or configuration; cf. EP 2 892 663 B1. This also includes, in particular, surfaces having a (quasi-)stochastic appearance, which, however, are applied by means of a deterministic texturing method and which therefore are composed of deterministic shape elements. The surface structure implemented is more particularly a continuous peak region with a plurality of recurring valley regions, each joined to the peak region by flank regions.

A sheet metal is to be understood as referring generally to a flat metal product, which may be provided in sheet form or in blank form or in strip form.

Further advantageous embodiments and developments are apparent from the following description. One or more features from the claims, the description or else the drawing may be linked with one or more other features therefrom to form further embodiments of the invention. It is also possible for one or more features from the independent claims to be linked by one or more other features.

According to one embodiment of the sheet metal of the invention, the substructure is configured in a crystal-like manner in the peak region and/or in the valley region. The crystal-like configuration may be implemented lengthwise and/or spherically and/or ovally as an elevation and/or an indentation in the peak region and/or valley region, with the establishment more particularly of a length, width or diameter of the crystal-like configuration of between 0.5 and 20 μm, more particularly between 0.9 and 15 μm, preferably between 1.2 and 10 μm.

According to one embodiment of the sheet metal of the invention, the sheet metal is coated with a metallic coating. The sheet metal may be coated with a zinc-based coating which is applied by hot-dip coating. More particularly the sheet metal is a sheet steel. In addition to zinc and unavoidable impurities, the coating may preferably comprise elements such as aluminum with an amount of up to 5 wt % and/or magnesium with an amount of up to 5 wt % in the coatings. Sheet steels with zinc-based coating exhibit very good cathodic corrosion resistance, which has been used in automobile construction for years. Where improved corrosion resistance is envisaged, the coating additionally comprises magnesium with an amount of at least 0.3 wt %, more particularly of at least 0.6 wt %, preferably of at least 0.9 wt %. Aluminum may be present alternatively or additionally to magnesium, with an amount of at least 0.3 wt %, in order in particular to improve attachment of the coating to the sheet steel and more particularly substantially to prevent diffusion of iron from the sheet steel into the coating when the coated sheet steel is heat-treated, so that the positive corrosion properties continue to be maintained. The thickness of the coating in this case may be between 1 and 15 μm, more particularly between 2 and 12 μm, preferably between 3 and 10 μm. Below the minimum limit, adequate cathodic corrosion resistance may not be ensured, and above the maximum limit, there may be joining problems when the sheet steel of the invention or a component fabricated from it is joined to another component; in particular, if the maximum limit indicated for the thickness of the coating is exceeded, it is not possible to ensure a stable process during thermal joining or welding. In the case of hot-dipped coating, the sheet steels are initially coated with such a coating and then passed on for skin-pass rolling. The skin-pass rolling takes place after the hot-dip coating of the sheet steel.

Alternatively the sheet metal may be coated with a metallic coating, more particularly a zinc-based coating, applied by electrolytic coating. In this case the thickness of the coating may be between 1 and 10 μm, more particularly between 1.5 and 8 μm, preferably between 2 and 5 μm. In comparison to the hot-dip coating, the sheet steel can first be skin-pass rolled and then coated electrolytically. Depending on the thickness of the coating, the roughness in the flank region can be substantially retained even after the electrolytic coating. An alternative possibility is initially an electrolytic coating with subsequent skin-pass rolling.

It is also conceivable for no coating to be provided, such as no metallic coating, for example. It is also conceivable for the sheet metal to have been/be coated with a nonmetallic coating, in a coil coating facility, for example, with the sheet metal being skin-pass rolled before or after it is coated with a nonmetallic coating.

According to one embodiment of the sheet metal of the invention, the sheet metal is coated with a phosphate coating or silane-based coating, more particularly the thickness of the phosphate coating or silane-based coating being less than 500 nm. In order nevertheless to be able to retain the advantages of a (phosphate) coating, more particularly in terms of the wetting behavior and/or as an adhesion base for coating-material coating and/or adhesive systems, and to be able to continue to employ existing process routes designed for phosphate sheet metals, the sheet metal may be coated with a phosphate coating or with a silane-based coating. The thickness of the phosphate coating or silane-based coating may be established at less than 500 nm, more particularly less than 200 nm, preferably less than 100 nm, more preferably less than 50 nm, very preferably less than 25 nm. Conventional zinc phosphating forms a coating on the surface of the sheet metal that has a thickness of at least 500 nm and that is insulating, electrically nonconducting, and may therefore have a process-disrupting effect in the case of a welding process, particularly a resistance welding process. Reducing an insulating, electrically nonconducting phosphate coating or silane-based coating to a thickness below 500 nm does not represent a process-disrupting factor.

According to a second aspect, the invention relates to a method for producing a formed and coated sheet-metal component, the method comprising the following steps:

-   -   Providing a sheet metal having a deterministic surface         structure, the surface structure having been impressed into the         sheet metal by means of a skin-pass roll, the surface structure         having at least one peak region and at least one valley region,         the peak region and the valley region being joined by a flank         region,     -   forming the sheet metal into a sheet-metal component,     -   coating the formed sheet-metal component.

In accordance with the invention the skin-pass roll with which the surface structure has been impressed into the sheet metal, during the impressing in the peak region and/or in the valley region, has generated a substructure such that a substructure has been generated that has a surface area greater by at least 3% in comparison to a flat projection face of the peak region and/or of the valley region or that has an Sdr of at least 3%.

Depending on the sheet-metal component to be produced, a corresponding sheet metal is provided more particularly that is cut before, during and/or after the forming. The forming takes place with conventional tools according to implementation.

The coating of the formed sheet-metal component takes place in a conventional way.

In order to avoid repetition, reference is made respectively to the observations made in respect of the sheet metal of the invention.

At least one valley region may be configured as an open structure on a skin-pass roll. Peak regions on the skin-pass roll therefore define local and recurring elevations on the surface of the skin-pass roll. By corresponding action of the skin-pass roll on a surface of a sheet metal, the peak regions of the skin-pass roll are impressed into the surface of the sheet metal and form a surface structure having a substantially closed structure (closed volume). The peak regions of the skin-pass roll therefore generate pocketlike structures on the surface of the sheet metal. The closed volume, referred to as the empty volume, is able to accommodate a process medium applied for later processing by means in particular of forming processes, such as forming oil, for example. Moreover, in the at least one valley region and/or in the peak region or peak regions of the skin-pass roll, a (negative) substructure is formed which by action on the surface of the sheet metal generates a (positive) substructure having a surface area greater by at least 3% in comparison to a flat projection face of the peak region and/or of the valley region, or having an Sdr of at least 3%.

The generation of a deterministic surface topography with at least one peak region or peak regions and at least one valley region, including (negative) substructure, on the surface of the skin-pass roll may be accomplished in a targeted way by means of a laser texturing process—cf. EP 2 892 663 B1.

The geometric configuration (size and depth) of the deterministic surface topography in the form of at least one peak region or peak regions and at least one valley region, including (negative) substructure, may be brought about individually through the use of a pulsed laser, as a result of depletion of material on the surface of the skin-pass roll. It is possible in particular through targeted actuation of the energy and of the pulse duration of a laser beam acting on the surface of the skin-pass roll to exert influence positively on the design of the structure or structures. With a high or higher pulse duration, the time of interaction between laser beam and skin-pass roll surface goes up, and more material can be ablated on the surface of the skin-pass roll. On the surface of the skin-pass roll, a pulse leaves behind a substantially circular, more particularly concave, crater, which after skin-pass rolling models the surface of the sheet steel. A reduction in the pulse duration has an influence on the formation of a crater; more particularly, the diameter of the crater can be reduced. By reducing the pulse duration, especially when using short-pulse or ultrashort-pulse lasers, it is possible to tailor the geometric structure on the surface of the skin-pass roll in such a way as to be able to provide a sheet steel surface with functionally appropriate texturing. This is achieved, for example, if the pulse duration of the laser with which the surface of the skin-pass roll is textured is reduced and it is possible accordingly to generate the geometric structure on the roll with higher resolution.

According to one embodiment of the method of the invention, no zinc phosphating has been carried out prior to the forming of the sheet metal. As a result of the invention, the costly and inconvenient step of conventional zinc phosphating for the purpose of generating a greater surface area by means of zinc phosphate crystals can be substantially omitted.

According to one embodiment of the method of the invention, the sheet metal was coated with a phosphate coating or silane-based coating before the sheet metal was provided, more particularly the thickness of the coating being less than 500 nm. In this way it is possible to retain the advantages of a phosphate coat and in particular, by reduction in the coating thickness, to implement preferably resistance welding in a reliable process. The phosphating encompasses, in particular, the deposition/laying-down of surfactants, a conversion chemistry or pickling with phosphoric acid, for example.

According to one embodiment of the method of the invention, the sheet metal has been treated with an acidic solution before or after the surface structure was introduced. The use of an “acidic” solution, which has a pH of less than 3, more particularly less than 2, preferably less than 1, is used preferably for the cleaning of the surface and/or for the removal of oxide adhesions (oxide coat) on the surface of the sheet metal.

According to one embodiment of the method of the invention, the sheet-metal component is an outer skin component of a vehicle. Outer skin components in particular are subject to stringent requirements in terms of forming suitability and coating-material appearance. As a result of the invention, corresponding outer skin parts can be produced inexpensively.

According to an alternative embodiment of the method of the invention, the sheet-metal component is a structural part of a vehicle.

Specific embodiments of the invention are elucidated in more detail below with reference to the drawing. The drawing and accompanying description of the resulting features should not be read as limiting on the respective embodiments, instead serving to illustrate exemplary embodiments. Moreover, the respective features may be utilized with one another and with features from the description above for possible further developments and improvements of the invention, especially in the case of the additional embodiments which are not represented.

Identical parts are always given the same reference symbols.

In the drawing

FIG. 1 ) shows a schematic view in partial section of one embodiment from the prior art,

FIG. 2 ) shows a schematic view in partial section of one embodiment according to the invention,

FIG. 3 ) shows a schematic view in partial section of another embodiment according to the invention, and

FIG. 4 ) shows a schematic sequence of one embodiment according to a method of the invention.

FIG. 1 shows a schematic view in partial section of one embodiment from the prior art. The embodiment may correspond, for example, to the embodiment according to EP 2 892 663 B1. A sheet metal (1) is represented which has a deterministic surface structure (2), the surface structure (2) being impressed into the sheet metal (1), with the surface structure (2) having at least one peak region (1.1) and at least one valley region (1.2), the peak region (1.1) and the valley region (1.2) being joined by a flank region (1.3). The sheet metal (1) is preferably a sheet steel.

FIG. 2 shows a schematic view in partial section of one embodiment according to the invention. In contrast to FIG. 1 , the peak region (1.1) and/or the valley region (1.2) has a substructure (1.11, 1.21) which is configured such that the substructure (1.11, 1.21) has a surface area greater by at least 3% in comparison to a flat projection face (P) of the peak region (1.1) and/or of the valley region (1.2), or has an Sdr of at least 3%. The substructure (1.11, 1.21) may be configured in a crystal-like manner in the peak region (1.1) and/or in the valley region (1.2), with the crystal-like configuration being implemented lengthwise and/or spherically as an elevation and/or indentation, shown as an indentation in this embodiment, in the peak region (1.1) and/or valley region (1.2); it is possible in particular to set a length, width or diameter of the crystal-like configuration of between 0.5 and 20 μm.

FIG. 3 shows a schematic view in partial section of another embodiment according to the invention. In comparison to FIG. 2 , the sheet metal (1) is coated with a metallic coating (3), preferably with a zinc-based coating. Alternatively or, preferably, additionally the sheet metal (1) is coated with a phosphate coating (4), in which case the thickness of the phosphate coating (4) may be less than 500 nm.

A sheet metal (1) of the invention, more particularly in accordance with the embodiment in FIG. 3 , is provided for the purpose of producing a formed and coated sheet-metal component not shown, (A). In a subsequent step, the sheet metal (1) provided is formed into a formed sheet-metal component, (B). After forming has taken place, the formed sheet-metal component is coated, (C). FIG. 4 shows schematically a corresponding sequence of the method of the invention. The formed and coated sheet-metal component not shown may be used as an outer skin part or structural part in the vehicle.

The individual features can all be combined with one another in so far as is technically possible. 

1. A sheet metal having a deterministic surface structure, the surface structure being impressed into the sheet metal, the surface structure having at least one peak region and at least one valley region, the peak region and the valley region being joined by a flank region wherein at least one of the peak region and the valley region have a substructure configured such that the substructure has a surface area greater by at least 3% in comparison to a flat projection face (P) of at least one of the peak region and of the valley region or has an Sdr of at least 3%.
 2. The sheet metal as claimed in claim 1, wherein the substructure has a surface area greater by at least 7% in comparison to the flat projection face (P) of at least one of the peak region and of the valley region or has an Sdr of at least 7%.
 3. The sheet metal as claimed in claim 2 wherein the substructure being configured in a crystal-like manner in at least one of the peak region and in the valley region.
 4. The sheet metal as claimed in claim 3 wherein, the sheet metal is coated with a metallic coating.
 5. The sheet metal as claimed in claim 4 wherein, the sheet metal is coated with one of a phosphate coating or silane-based coating, and wherein the thickness of the coating is less than 500 nm.
 6. A method for producing a formed and coated sheet-metal component, the method comprising the following steps: providing a sheet metal having a deterministic surface structure, the surface structure having been impressed into the sheet metal by means of a skin-pass roll, the surface structure having at least one peak region and at least one valley region, the peak region and the valley region being joined by a flank region, forming the sheet metal into a sheet-metal component, coating the formed sheet-metal component, wherein the skin-pass roll with which the surface structure has been impressed into the sheet metal, during the impressing in at least one of the peak region and in the valley region, has generated a substructure such that a substructure has been generated that has a surface area greater by at least 3% in comparison to at least one of a flat projection face (P) of the peak region and of the valley region or that has an Sdr of at least 3%.
 7. The method as claimed in claim 6, wherein no zinc phosphating having been carried out before the forming of the sheet metal.
 8. The method as claimed in claim 7, wherein the sheet metal, before the providing of the sheet metal, has been coated with one of a phosphate coating or silane-based coating, wherein the thickness of the coating is less than 500 nm.
 9. The method as claimed in claim 8, wherein the sheet metal has been treated with an acidic solution before or after the introduction of the surface structure.
 10. The method as claimed in claim 9, wherein the sheet-metal component is an outer skin part of a vehicle.
 11. The method as claimed in claim 9, wherein the sheet metal component is a structural part of a vehicle. 